Cold cathode tubes have conventionally been used as backlights of liquid crystal display devices. In addition to high brightness and high efficiency, a long life is required of cold cathode tubes in such application. For example, in the application in a liquid crystal TV and the like, they are required to have a long life of more than 60000 hours. Further, since the discharge starts with glow discharge in cold cathode tubes, a material more easily causing the start of the discharge is desired for electrodes for cold cathode tube.
A cold cathode tube useful as a backlight for liquid crystal display device has a structure such that minute amounts of mercury and rare gas are filled in a glass tube whose inner surface is coated with a phosphor, and electrodes for cold cathode tube and lead-in wires (for example, KOV+dumet wires) are mounted in both end portions of the glass tube. In such a cold cathode tube, upon voltage application to the electrodes in the both ends, the mercury sealed in the glass tube is evaporated, resulting in emission of ultraviolet light, and the phosphor absorbs the ultraviolet light to emit light.
As a material forming electrodes for cold cathode tube, a material which is relatively easily worked, such as Ni, Ti, or Al, is used. Such an electrode for cold cathode tube, however, is likely to deteriorate the life of a lamp because of a relatively high cathode fall voltage necessary for electron emission from the electrode to a discharge space and because of the occurrence of what is called a sputtering phenomenon. The sputtering phenomenon refers to a phenomenon that an electrode material scatters when ions collide with the electrode during lighting of the cold cathode tube, and the scattered material, the mercury, and the like deposit on a wall surface in the glass tube.
The mercury is taken into a deposit layer formed by the sputtering phenomenon, and thus is not usable for light emission, which extremely lowers brightness of the cold cathode tube. Reducing such a sputtering phenomenon reduces the consumption of the mercury, realizing a longer life even with the same amount of the sealed mercury.
In order to lower the cathode fall voltage and reduce the sputtering phenomenon, an electrode structure has been attempted in which an electrode for cold cathode tube is formed in a bottomed cylindrical shape and a cutout portion is provided in its sidewall portion (see a Patent Reference 1). This structure aims at both the effect of lowering the cathode fall voltage and the effect of reducing the sputtering, by a hollow cathode effect.
Further, there has been made a proposal of a cylindrical electrode for cold cathode tube made of tungsten or the like, whose surface area is increased by setting a relation between a thickness a of its bottom portion and a thickness b of its side surface portion to a>b and by using a sintered compact with a 80 to 98% relative density (see a Patent Reference 2). With these designs, it has been attempted to increase a coverage amount of an electron emissive material to prolong the life of the cold cathode tube.
The electrodes for cold cathode tubes are modified in their shapes and the like, thereby realizing a longer life. However, the conventional electrodes for cold cathode tube are facing a demand for further improvement in characteristic though a certain level of improvement is recognized therein regarding a discharge start characteristic, a life characteristic, and the like. Further, the electrodes for cold cathode tube are manufactured by, for example, using a drawn metal plate. However, conventional metal plates used for manufacturing the electrodes have a problem that those excellent in workability are inferior in discharge start characteristic and the like, whereas those excellent in discharge start characteristic are inferior in workability.
Patent Reference 1: JP-A 2001-176445 (KOKAI)
Patent Reference 2: JP-A 2004-178875 (KOKAI)